Filter HTML _after_ Markdown to HTML conversion #2216
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(Description added post-merge.)
Looks like my previous PR introduced a security vulnerability that resulted from a poor order of operations when converting Markdown to HTML. The original PR filtered out undesirable HTML from raw Markdown before converting the result to HTML. Carefully crafted Markdown can be written to take advantage of this to run arbitrary JS, and likely do other undesirable things.
This PR reverses the order of the conversion: first the raw Markdown is converted to HTML, and then the result is processed through the HTML filter. In this way, we block this attack vector. Thanks to @rafl for pointing out the issue! (And apologies for creating the security hole in the first place.)